1. Field of the Invention
This invention relates to a liquid crystal display device, specifically to a liquid crystal display device in which an alignment direction of liquid crystal molecules is controlled by a lateral electric field between electrodes on the same substrate.
2. Description of the Related Art
As a way to achieve a wide viewing angle of the liquid crystal display device, a method has been developed to realize a light switching function by rotating the liquid crystal molecules in a plane parallel to the substrate with a lateral electric field generated between the electrodes on the same substrate. In-Plane Switching (hereafter referred to as IPS) method and Fringe-Field Switching (hereafter referred to as FFS) method are known as examples of these technologies.
Next, the liquid crystal display device according to the FFS method is explained referring to the drawings. FIG. 12 is a plan view showing the liquid crystal display device according to the FFS method. FIG. 13 is a cross-sectional view of a section Y-Y in FIG. 12. Although a plurality of pixels is disposed in a matrix form in the actual liquid crystal display device, only one pixel is shown in these figures.
A TFT substrate made of a glass substrate or the like faces a light source BL. A first polarizing plate 11 that linearly polarizes light from the light source BL is formed on a surface of a side of the TFT substrate 10 facing the light source BL. A buffer film 12 made of a silicon dioxide film, a silicon nitride film or the like is formed on a surface of the opposite side of the TFT substrate 10.
An active layer PS made of polysilicon or the like is formed on the buffer film 12 in an area where a thin film transistor TR for pixel selection is to be formed. A gate insulation film 13, that is made of a silicon dioxide film, a silicon nitride film or the like and covers the active layer PS, is formed on the buffer film 12. A gate line GL is formed on the gate insulation film 13 so as to face the active layer PS. The gate line GL is made of a metal including chromium or molybdenum or the like. A common electric potential line COM, that is made of a metal including chromium or molybdenum or the like and provides a common electric potential, is formed on the gate insulation film 13. The gate line GL, the common electric potential line COM and the gate insulation film 13 are covered by an interlayer insulation film 14.
The interlayer insulation film 14 is provided with a contact hole CH1, that exposes a source region in the active layer PS, and a contact hole CH2, that exposes a drain region in the active layer PS. The interlayer insulation film 14 is also provided with a contact hole CH3 that exposes the common electric potential line COM.
A display signal line DL, that is connected with the source region in the active layer PS through the contact hole CH1, is formed on a surface of the interlayer insulation film 14. A drain electrode 15, that is connected with the drain region in the active layer PS through the contact hole CH2, is formed on the surface of the interlayer insulation film 14. Also, a pad electrode 16, that is connected with the common electric potential line COM through the contact hole CH3, is formed on the surface of the interlayer insulation film 14. The display signal line DL, the drain electrode 15 and the pad electrode 16 are made of a metal including aluminum or aluminum alloy or the like. The display signal line DL, the drain electrode 15, the pad electrode 16 and the interlayer insulation film 14 are covered by a passivation film 58.
A planarization film 59 is formed on the passivation film 58. The passivation film 58 and the planarization film 59 are provided with a contact hole CH5, that exposes the drain electrode 15, and a contact hole CH6, that exposes the pad electrode 16.
A pixel electrode 60, that is made of a first layer transparent electrode such as ITO (Indium Tin Oxide) and is connected with the drain electrode 15 through the contact hole CH5, is formed on the planarization film 59. A display voltage corresponding to a display signal is applied to the pixel electrode 60. An insulation film 61 is formed on the pixel electrode 60 to cover it. A common electrode 62, that is made of a second layer transparent electrode such as ITO and has a plurality of slits S extending parallel to each other, is formed on the insulation film 61. The common electrode 62 is connected with the pad electrode 16 through the contact hole CH6. An alignment film (not shown) covering the common electrodes 62 is formed over the insulation film 61.
A color filter substrate (hereafter referred to as CF substrate) 20 made of a glass substrate or the like faces the TFT substrate 10. A color filter and an alignment film (not shown) are formed on a surface of a side of the CF substrate 20, which is facing the TFT substrate 10. A second polarizing plate 21 is formed on a surface of another side of the CF substrate 20, which is not facing the TFT substrate 10. The first and second polarizing plates 11 and 21 are positioned so that their polarization axes are perpendicular to each other. A liquid crystal 30 is sealed between the TFT substrate 10 and the CF substrate 20.
In the liquid crystal display device described above, an average alignment direction (hereafter simply referred to as “alignment direction”) of major axes of the liquid crystal molecules of the liquid crystal 30 is parallel to the polarization axis of the first polarizing plate 11 when the display voltage is not applied to the pixel electrode 60 (no voltage state). In this case, the linearly polarized light passing through the liquid crystal 30 does not go through the second polarizing plate 21 because its polarization axis is perpendicular to the polarization axis of the second polarizing plate 21. That is, black is displayed (normally black).
When the display voltage is applied to the pixel electrode 60, on the other hand, there are caused electric fields from the common electrode 62 toward the underlying pixel electrode 60 through the slits S. (Refer to arrows in FIG. 13.) The electric fields are perpendicular to a longitudinal direction of the slits S on the two-dimensional view, and the liquid crystal molecules rotate along lines of electric force of the electric fields. (Refer to arrows in FIG. 12.) At that time, the linearly polarized incident light to the liquid crystal 30 is turned into elliptically polarized light by birefringence to have a component of linearly polarized light that passes through the second polarizing plate 21. In this case, white is displayed. The liquid crystal display device according to the FFS method is disclosed in Japanese Patent Application Publication No. 2002-296611, for example.
When the display voltage is applied to the pixel electrode 60, while the lines of electric force perpendicular to the longitudinal direction of the slits S are caused at central regions SC of the slits S shown in FIG. 12 as described above, the lines of electric force of different directions are caused at edges SE of the slits S. The alignment direction of the liquid crystal molecules is different from that in the central region SC of the slits. Portions of the liquid crystal of the different alignment direction make domains (portions indicated by circles in FIG. 12). Such a fault in alignment of the liquid crystal molecules is called disclination. Due to the disclination, there has been a problem of degradation in optical characteristics, that is, localized reduction in transmittance and contrast in the normally black.